My betta got fin rot for the first time ever about a week ago. It was very fast moving, his fins were a bit tattered in the morning, and by the evening, looked like they got attacked! I cleaned his tank, a full water change, and treated the fresh water with BettaSafe as usual. His fins started to heal, but two days later started to tatter again.

I went out and got T.C. Tetracycline to treat him. Now the finrot is completely gone, and hes healthy but the medication made his water a nasty shade of brown and I'm all out of filters till this coming friday when i get paid.

My concern is what caused his finrot? I'm a college student, and my roommates betta got sick, and died for unknown reasons a week before my betta got finrot. She then bought a new baby betta halfmoon, who within a day got finrot, right around when mine got finrot. She has also since treated him, and hes healing.

My roomate has also taken to filling her tank with jugs of water she buys. From what I've heard though this is bad because of the mineral deficiencies of bottled water. However, she brought up the idea that maybe the pipes in our dorm are old and are what is giving our bettas finrot. My fish has never had finrot before
till i moved in here and cleaned the tank. Even though I treat the water with BettaSafe could this be the cause of the rot? If so, what can I do? It's the only water I have access to.

I need to clean his tank because the water is brown from the medication, but I'm afraid the fresh water will make his fins rot again! Help!

When you found his fins in tatters or when you described him as appearing attacked, he was likely chewing his own fins.
I've got a fin-chewer with early stages of fin rot as well. You aren't alone!
I don't know why a fish chews his fins. Sometimes they are stressed. Some people suggest that they're hungry (which I find can be true depending on the fish). Sometimes their fins get too long and heavy and they chew them shorter. Or sometimes, the fish is just bored! It depends on the individual fish's personality.
Strange, isn't it?
I highly doubt anything you're adding to the water is causing the fin rot. It typically happens in filthy water, or water in need of a change.
Not all bottled water is deficient in minerals. Distiller water is, and you NEVER want to use it for a fish. Spring water is perfectly fine, and I use it all the time.
Truth be told, I don't think changing his water will cause a reoccurrence of fin chewing and fin rot.
I do think your fish might feel stressed. Especially if he's new to his tank or if there is a significant tank size difference. If you haven't already, try adding an ornament for him to hide in (my fish love theirs- they sleep in them at night and are social during the day). Maybe even a plant- live or plastic.
In the meantime, I would use a little bit of Epsom salt. Fill a gallon jug with your water (or buy spring water) and dissolve 1 1/2 or 2 tsp Epsom salt. This may help relax him a bit. If you do choose to use Epsom salt, take out the filter cartridge, so it doesnt remove the salt.

I really do think your fish will be fine. If he really is a fin-chewer, he will likely stop his habit and thus never have to deal with fin rot again.